From the entire periodic table, there are only a few elements with a stable and biocompatible oxidation state and a high number of unpaired electrons that are considered suitable for image enhancement applications in magnetic resonance imaging (MRI). These include Mn(II) (HS; S=5/2), Fe(III) (HS; S=5/2) and Gd(III) (S=7/2). The MRI signal intensity (SI) from different body tissues varies with the content of water protons present in the tissue and with both the longitudinal (T1) and transverse (T2) relaxation times of those protons. In some cases, the variation of water content in different tissues is sufficient to produce image contrast. In other cases, it is necessary to use a contrast agent to enhance the image contrast. Chemical compounds that can change the relaxation times, either T1 or T2, within a tissue are routinely used as contrast agents in MRI in the medical diagnosis of diseases and/or organ functions in the human body.
Clinical MRI contrast agents can be divided into two classes, T1 agents and T2 agents. A T1 or positive contrast agent shortens the longitudinal relaxation time (T1) of water protons, and can brighten regions where the agent is present. Conversely, a T2 or negative contrast agent reduces the transverse relaxation time (T2) of water protons, and produces darkened spots in the tissues reached by the agent when a residual transverse magnetization is used in a spin-echo experiment.
The majority of the T1 contrast agents have been developed from the use of the paramagnetic Gd3+ ion chelated by various low molecular weight polyaminopolycarboxylate ligands. The high electronic spin (4f7, S=7/2, 7.9 BM), coupled with a symmetric electronic ground state (8S7/2) and slow electronic relaxation (10−9 s), gives Gd(III) unique nuclear-magnetic properties for enhancing T1-relaxation of protons from bulk water. Currently, there are nine commercial T1 agents approved worldwide for clinical use (FIG. 1 and Table 1).
The major drawback of these agents is their limited sensitivity (relaxivity). The relaxivity of a contrast agent is the measure of its efficacy and usually expressed as the concentration-normalized amount of increase in the longitudinal relaxation rate 1/T1 per millimole of agent in the unit of mM−1×s−1. As a result, MR imaging applications using such agents require high tissue concentrations (0.1-0.6 mM). Relaxivity of these agents drops significantly at higher magnetic fields, which makes them inefficient in the high-field MR scanners for clinical diagnostic imaging. The high-field scanners can greatly shorten data acquisition time, improve signal-to-noise ratio (SNR) and provide higher spatial resolution. Recently, the use of the Gd3+-based MRI contrast agents has been linked to nephrogenic systemic fibrosis (NSF), an acute and fatal toxic adverse reaction in patients with impaired renal function. NSF is believed to be caused by the in vivo release of the Gd3+ ions from the chelates. The toxicity of gadolinium stems from the fact that the ionic radius of Gd(III) (1.02 Å) is very similar to that of calcium(II) (1.00 Å). Hence, the presence of this heavy metal ion in the body can disrupt the normal functions of many types of voltage-gated Ca2+-channels at the nano- to micro-molar concentration level. In addition to toxicity, due to the lack of ability to penetrate cells, these small molecule-based T1 contrast agents function only as extracellular agents, which limits their use in detecting biological receptors or markers within the cell and makes them ineffective as cellular MR probes.
TABLE 1Stability constants and relaxivity valuesfor the commercial MRI contrast agentsTrademarkLogKGdLr1(mM−1 × s−1)Dotarem ®25.34.2ProHance ®23.84.4Gadovist ®20.85.3Magnevist ®22.24.3Omniscan ®16.84.6OptiMARK16.85.2MultiHance18.46.7Primovist23.57.3Vasovist23.219
All the existing T2 contrast agents are based on superparamagnetic iron oxide nanoparticles (SPIOs). These agents shorten the transverse relaxation time (T2) of bulk water to produce a negative or darkened contrast. Although SPIOs are nontoxic and FDA-approved contrast agents with higher sensitivity and can penetrate cells, from the standpoint of clinical diagnosis and cellular imaging, the image contrast produced by such agents is far less desirable than that by the T1 agents. It is difficult to distinguish between the darkened spots produced by the accumulation of a T2 agent and the signals caused by bleeding, calcification, metal deposit, or other artifacts from the background. This fact can complicate the correct interpretation of imaging results, and is a major barrier for T2 agents to gain widespread clinical applications in replacement of T1 agents. Besides this, imaging with T2 contrast requires longer acquisition times. Currently, the primary application of SPIOs T2 agents is for image-guided drug delivery and the monitoring of surgical procedures.
Besides the Gd3+ ion of seven unpaired electrons, the next highest possible number of unpaired electrons is five (S=5/2). The electron configuration corresponding to this spin state is found in the stable transition metal ions Fe(III) and Mn(II). Although iron is an essential element in biology, the use of analogous Fe3+-chelates to deliver Fe(III) for T1 MRI contrast is deemed unacceptable due to the high cellular toxicity of this metal. Because most high-spin Fe(III) complexes have low to modest thermodynamic stability and are kinetically labile, in vivo release of free Fe3+ ions from such chelates is inevitable. As the result, any Fe3+-containing compound administered parenterally can disturb the iron homeostasis that is tightly regulated by ferritin and transferrin receptors in the body. The ferrous ion Fe2+, produced from any non-sequestered ferric ion through reduction by a variety of biomolecules, can catalyze the generation of reactive oxygen species (ROS) including hydroxyl radical and peroxide radical via the so-called Fenton chemistry:Fe2++H2O2→Fe3++OH.+OH−  (1)Fe3++H2O2→Fe2++OOH.+H+  (2)
The above ROS species can lead to wide-spread systemic injury to the liver, heart and endocrine organs as well as increases in infection. To avoid the Fe3+ or Fe2+ ions to be leached into the body, completely insoluble iron compounds in the form of superparamagnetic iron oxide nanoparticles (SPIOs of Fe3O4 or γ-Fe2O3) have been developed as T2 MRI contrast agents (vide supra).
An FDA approved Mn2+-based small-molecule complex has been developed as a MRI contrast agent, namely manganese dipyridoxal diphosphate (MnDPDP) for application to liver, pancreas, and heart. However, the Mn2+ is shown to be released in vivo due to the transmetallation with zinc(II). Therefore, the contrast enhancement detected in these organs is due to the presence of the released paramagnetic Mn2+ ions. The cellular toxicity of higher level manganese (>1 mM) has prevented any Mn2+-complex from being developed as the generalized MRI contrast agent. It is well known that exposure to high concentration level of Mn2+ can lead to neurological deficits, particularly a neurological disorder resembling Parkinson's disease.